For many injection molding applications, it is desirable to position the mold cavities as close together as possible, to maximize the number of articles that can be molded using a given size of injection molding machine. For molded articles that are small, such as contact lenses, the nozzles on a typical injection molding machine may be a limiting factor in terms of how closely spaced the mold cavities can be. This is because the nozzles themselves may be too wide to position as close together as would be desirable.
One approach to permitting a small mold cavity-to-mold cavity spacing is to use multi-tip nozzles. A multi-tip nozzle is a nozzle that has a primary melt channel that receives melt and that divides into several secondary melt channels. The nozzle has several tips at its end, through which the secondary melt channels exit into several mold cavities. Several types of multi-tip nozzles are known.
An example of such a multi-tip nozzle is shown in U.S. Pat. No. 5,268,184 (Gellert). Gellert discloses a multi-tip nozzle having a forward portion with a central melt channel that divides into several smaller melt channels. The forward portion has several tips out of which melt flows into several mold cavities. At each tip, a rod made from a wear resistant material is inserted into a machined bore and welded to the rest of the forward portion.
A separate but related issue exists with nozzles, and particularly small-pitch nozzles, on typical injection molding machines. Typically, an injection molding machine includes a mold cavity block that is maintained at a lower temperature than the nozzle for at least a portion of an injection molding cycle. A downstream portion of the nozzle is usually proximate and may even contact the mold cavity block. As a result, the nozzle may be subject to the cooling effects of the mold cavity block. Thus, the temperature profile of melt in a typical nozzle of the prior art shows that the temperature of the melt is lower at the downstream end of the nozzle body and higher in the middle region. In order to heat the downstream end of the nozzle it is desirable to position the heater as close as possible to the downstream end, however space constraints may prevent the heater from being positioned as far downstream on the nozzle as would be desirable to counteract the heat loss into the mold cavity block. Such space constraints may be worse for a multi-tip nozzle than for a typical single-tip nozzle.
Furthermore, nozzles on injection molding machines are often immediately downstream from a manifold block, which transfers melt to the nozzles from a melt source. Typically, the manifold block is maintained at a lower temperature than the nozzle, and as a result, the nozzle can lose heat into the manifold block at its upstream end.
As a result, the temperature profile of a typical nozzle shows a maximum temperature in a middle region of the nozzle, and lower temperatures at the upstream and downstream ends.
Depending on the configuration of the nozzle and the particular material being injection molded, it may be difficult to heat the nozzle in such a way as to have melt in the downstream end of the nozzle at an acceptably high temperature to penetrate the mold cavity fully, without burning the melt in the middle region of the nozzle. Conversely, reducing the temperature of the melt in the nozzle to ensure that the melt in the middle region doesn't burn, may cause the melt at the downstream end to become too cold to flow as desired into the mold cavity. Other problems can also occur with the melt if it is too hot or too cold also.